Blade Comprising A Support, Provided With A Portion With A Depression

Abstract

The support (6) of this blade comprises one side (7) and the other (18) provided with a depression (10). By concentrating this depression on a single side, a support (6) that is more rigid and resistant with an equal lightening is obtained, with the stress concentrations depending on the depth of the depression but being absent from the flat side (7). The vibrations and bending deformations are also reduced. Application to blades of a turbomachine, in particular compressors.

Description

[0001] This invention relates to a blade comprising a support provided with a portion with a depression on a single side.

[0002] This blade, which can be a turbomachine compressor blade, comprises an outside vane subjected to aerodynamic forces, and a foot retained in a rotating disc; the foot comprises a platform flush with the surface of the disc and from which the vane rises, a bulb retaining the blade against the centrifugal forces and retained in an enlargement of a groove of the disc, and a support joining the bulb to the platform, and housed in a main portion of the groove.

[0003] Portions with a depression have sometimes been dug into the main or lateral faces of the support so as to lighten the blade. Disadvantages, such as local increases in the static stresses and an alteration in the frequency of the bending modes, have however been observed with respect to blades of which the supports are devoid of a depression.

[0004] In French patent application 12 60527, it was undertaken to reduce these disadvantages and to bring the behaviour of the blade provided with portions with a depression closer to that of a blade with perfectly flat supports, by choosing dissymmetric portions with a depression, in particular with regards to the depth. This application clearly specifies that the portions with a depression are established on the two main sides of the support.

[0005] Document U.S. Pat. No. 5,435,694 A describes a blade wherein the support is joined to the platform or to the bulb by fillets, which are mouldings with a slightly concave section, in order to reduce the stress that the blade in service is subjected to. Portions slightly with a depression are added in order to reinforce this effect, by decreasing the concentrations of stress around the fillets. These portions with a depression are defined by two successive radii of curvature and are superficial. It seems that they are provided on the two sides of the support. It is not sought to lighten the blade by means of these portions with a depression or by fillets.

[0006] GB 2 162 588 A describes a blade support provided with a recess that opens onto one of the end faces of the support in order to lighten it. This design is suitable for relatively thick supports. The structure of the support remains symmetrical in the angular direction on either side of a median plane extending in the length of the support.

[0007] The invention relates to a lightened blade, designed to minimise the degradations in aeromechanical performance, in particular concerning the concentrations of stress and the frequency of the bending modes.

[0008] According to the invention, one of the main faces of the support is substantially plane or curvilinear--in any case with a regular surface without relief--while the other of the main faces is provided with a portion with a depression, extending over a depth of at least 25%, and advantageously by at least 35%, of a thickness of the support, between the main faces. It is specified that the main faces of the support--which extends roughly in the axial direction of the disc and of the turbomachine to which the blade belongs--are the lateral faces with the largest surface area, roughly perpendicular to the angular direction of the disc and of the machine, giving onto the intrados and extrados sides of the blade, and extending in the direction of insertion of the foot of the blade into the cutout of the disc.

[0009] The degradation in the performance obtained on the blade is then clearly less substantial than with portions with a depression distributed symmetrically on the two sides of the support, or even than with the portions with a depression opposite and moderately dissymmetric considered in application FR 12 60527. And GB 2 162 588 does not suggest that a section of dissymmetric support in the angular direction would improve the rigidity of the support in this direction wherein the bending of the blade is exerted.

[0010] Indeed, flatness maintained on one of the main faces increases the rigidity of the blade with regards to the deformations in bending and prevents the introduction of concentrations of stress on one side: it can therefore be considered to arrange the depression on the side where the concentrations of stress are the least feared, i.e. the side that is the least loaded in the critical situations to be considered, such as the loss of a blade, in order to obtain a globally satisfactory distribution over the blade by not weakening the other side.

[0011] According to other optional improvements, the portion with a depression has a bottom that is substantially flat or curvilinear (parallel to the main face, and therefore at a uniform depth), surrounded by a curved edge, connecting to the bottom without forming an angle, in order to obtain a good lightening without favouring concentrations of stress at the edge of the portion; where it is limited by rectilinear sides joined by curvilinear connectors.

[0012] The invention shall now be described in liaison with the following figures:

[0013] FIG. 1 is a general view of the blade in the angular direction; and

[0014] FIG. 2 is a view of the foot of the blade in the axial direction.

[0015] FIG. 1 shows a blade in accordance with the invention, provided with a vane 1 to the exterior and with a foot 2 engaged in the rotating disc 3, with the foot 2 comprising a platform 4 joined to the vane 1, a bulb 5 opposite the platform 4, and a support 6, joining the two latter items. As the platform 4 is conical, the support 6 is trapezoidal.

[0016] The support 6 comprises two main faces 7 and 8 opposite, shown in FIG. 2, giving onto the intrados and extrados sides of the blade and extending roughly in the direction of the axis of rotation of the disc 3. The main face 7 is flat (smooth, linearly or curvilinearly flat), but the main face 8 is provided with a portion with a depression 10, taking a flat bottom 12 and an edge 14 connecting the bottom 12 to the main face 8. The bordure 14 is connected to the bottom 12 without forming any sharp edge, but with a progressive rounding. The portion with a depression 10 is of trapezoidal shape, as shown in FIG. 1, with a first longitudinal side 15, close to the bulb 5 and parallel to its junction with the support 6, a second longitudinal side 16 close to the platform 4 and parallel to the connection between it and the support 6, and two other sides 17 and 18, connecting the latter together and curvilinear at least at the ends.

[0017] The portion with a depression 10 can be present according to case on the intrados or the extrados side of the blade.

[0018] By making use of a depression on a single side of the support 6, the gain in mass is provided while still minimising the effect on the aerodynamic and mechanical behaviour of the blade: the static stress, the position of the first bending mode and the response of the first bending mode under substantial stress, are improved with respect to a blade comprising, for the same lightening, portions with a depression on the two sides.

[0019] In relation to such a blade comprising two portions with a depression, it has been observed, in a design in accordance with the invention, wherein the thickness of the support 6 (distance between the two main faces 7 and 8) was 15.4 mm and the portion with a depression had a depth of 5.4 mm, a static stress at the output of the blade, on its contact surfaces with the disc 3, reduced by 5% in the most loaded zone with respect to a blade having depressions on the two sides and a similar lightening; and for a first bending mode, the deformations measured in the support 6 were reduced by 30% following substantial stress such as floating (instability due to the interaction of the fluid with the structure of the vanes 1 which has for effect to amplify the vibration of the blades), ingestions of foreign bodies or the loss of a vane 1.

[0020] It is therefore possible to hope for a better resistance, or a longer resistance in the case of fatigue, of the foot 2, and better resistance of the vanes 1 with regards to vibrations. It is to be added that the frequency of the bending mode is not modified by the concentration of the lightening of a single side, in such a way that the invention does not require having to again optimise the blade with regards to these frequencies, once the optimisation has been carried out for blades with supports recessed on the two sides.

[0021] The same considerations apply to blade supports of which the main faces would be curvilinear instead of being flat: the portion with a depression would again be located on only one of its main faces and would be dimensioned in the same way.

Claims

1. Blade, in particular of a compressor, comprising a vane (1) and a foot (2), with the foot comprising successively, by moving away from the vane, a platform (4) joined to a support (6) and a bulb (5), with the support (6) comprising two main faces (7, 8) respectively directed in an intrados and an extrados side of the blade and parallel, characterised in that one of the main faces (7) is substantially plane or curvilinear, while the other of the main faces is provided with a portion with a depression (20) extending over a depth of at least 25% of a thickness of the support between the main faces.

2. Blade according to claim 1, characterised in that the portion with a depression extends over at least 35% of the thickness of the support.

3. Blade according to claim 1, characterised in that the portion with a depression has a bottom (12) that is substantially flat or curvilinear surrounded by a curved edge (14) that connects to the bottom without forming an angle.

4. Blade according to claim 2, characterised in that the portion with a depression has a bottom (12) that is substantially flat or curvilinear surrounded by a curved edge (14) that connects to the bottom without forming an angle.

5. Blade according to claim 1, characterised in that the portion with a depression is limited by rectilinear sides joined by curvilinear connectors.